M104 belt tensioners can be a pain. Sometimes there's nothing wrong with it.
One thing alot of shops (even the good ones) miss is the PIVOT BOLT. It's this giant 10mm hex-key bolt that goes through the center of the tensioner. THIS BOLT MUST BE LOOSENED 3-5 TURNS TO PROPERLY ADJUST TENSIONER.
I tensioned mine for more than 10 years without releasing the pivot bolt. It would still adjust, just hardly enough to replace/tighten the belt. Finally, my belt got to the point a month or 2 ago that it was so loose, no matter how much I adjusted the tensioning rod it wouldn't get tighter (or looser). So, tensioner is bad right?
Ordered replacement, disassembled entire front of car for replacement only to discover the pivot bolt. I never even knew it was there. I reassembled the whole thing (old tensioner still inside). Left the pivot bolt a little bit loose this time. Discovered I had stripped out a tiny section of my adjusting rod (because in the past I hadn't loosened pivot bolt). I pressed REALLY HARD to get the nut on the adjusting rod to go farter than the stripped point. Then, I discovered I could tension everything normally. My old, 1994 factory original tensioner was just fine, minus a squeeky pulley, a noisy shock, and a stripped tensioning rod. But since I had ordered all new stuff anyway, i decided to take it all back apart and replace it (electing to keep all the OLD parts as spares).
When removing the pivot bolt through the center of the tensioner I discovered it was extremely hard to turn. 10 years of tensioning the assembly without loosening this bolt had warped the bolt. In the process of SLOWLY getting it out, I broke it off in the hole. I had to spend 3-4 hours drilling it out. In the end I went up to a 1/2" bolt (due to the damage to the threads in the hole). I also had to use a 1/2" drill bit to carefully bore out the center of the new tensioner so the half inch bolt would fit. Reassembled without a problem.
So before you start thinking your tensioner is bad or a shop has told you so: check it out yourself. I know someone who paid $880 for this job on their C280. Not pretty. Loosen that pivot bolt and THEN try to adjust the tensioning rod. If there's already tension, then your tensioner is fine. You've got a pulley going out somewhere. 50% its the tensioner pulley. 49% the idler pullet (top passenger side of motor). 1% any other pulley.
1994 C280, green on tan, 240k mi. <--old faithful
1980 240D, white on blue, 140k mi.<--project (on hold until state allows me to drive it)
1994 E320, black on black, 130k mi.<--wife's
1999 C43 AMG, silver on black, 248k mi. <--daily